The SET DOWN and RESET DOWN options enable the database to attempt to avoid database crashes, whenever possible, by converting what would otherwise be a crash into a transaction abort and a snapshot dump for a selected set of normally fatal file system error events.
The database flags a table as being down when the maximum number of permissible down regions is exceeded on its data subtable. A table that is flagged down is no longer accessible for DML requests.
To mark only a portion of a table down, the database identifies a range of rows that it makes inaccessible to DML requests. This row range is called a down region and is also recorded in the table header. Any queries that do not access the down portions of the subtable continue to execute as they did before the region was flagged. Only requests that attempt to access rows in ranges that have been flagged as down abort their containing transaction.
In case the rollback processing for a transaction abort or system recovery attempts to process a row in a down region, the database ignores the rollback because the affected row set is inaccessible. Because ignoring the rollback creates a data integrity problem if the down region is later manually reset, the database flags down regions to indicate that rollback had been skipped for them.